As a food preservation method, freeze drying has several benefits over others. To maintain fresh flavor and nutritional value, frozen food requires uniform, below freezing storage conditions. High-temperature processing to produce dehydrated and canned foods degrades the flavor, texture, and nutritional content, despite producing a product that may be stored without refrigeration. Freeze drying preserves freshness, color, and aroma similar to frozen food without relying on refrigeration, like canned or dehydrated food. Freeze drying is intended to remove water from food, but not the flavor, color, texture or nutritional value.
Freeze dried foods may maintain their original shape and texture instead of shrinking or shriveling as in high-temperature processing for dehydration. Moisture channels and food fibers may remain intact so that, after just adding water, details of the fresh food return. Freeze dried foods may be light weight with at least 96% of their water removed and may be stored at room temperature without deterioration. Since freeze drying removes oxygen as well as water, foods packaged inside an oxygen barrier with an oxygen scavenger material may be stored long term. Individual items as well as prepared meals may be freeze dried.
Freeze drying, or lyophilization, involves freezing a material and then placing it under vacuum in a chamber at temperature and pressure conditions below the triple point of water (273.16 Kelvin, 611.73 Pascals), allowing frozen water in the material to sublimate to a gas, since it is below the triple point. As the term is used herein, the “triple point” refers to the temperature and pressure conditions at which solid, liquid, and gas phases are in equilibrium. Below the triple point of water, solid water does not transition to a liquid phase before becoming a gas. The solid to gas phase transition is referred to as “sublimation.”
A vacuum pump provides the vacuum pressure and an evaporator of a refrigeration system provides a surface colder than the material temperature whereon the gaseous water may condense and freeze to avoid moisture collecting in the vacuum pump. “Vacuum pressure,” as used herein, is an absolute pressure, unless otherwise indicated, that is below atmospheric pressure. Accordingly, a perfect vacuum exhibits zero pressure units (e.g., 0 milliTorr) of vacuum pressure and a pressure just below atmospheric pressure exhibits a higher number of pressure units (e.g., 759 milliTorr) of vacuum pressure with a positive value.
In known methods, chamber cooling maintains a constant chamber temperature. To accelerate sublimation, known methods heat a tray on which the material rests and the heating raises chamber vacuum pressure. Such known methods use process controls for cooling and vacuum pumping to maintain constant temperature and pressure. Vacuum pressure rises due to a water phase change from solid to gas and due to tray heating. The heating time, tray temperature, and ramping of tray temperature are material specific and are determined through trial and error to provide proper freeze drying for various materials according to designated process conditions. Examples of such known methods are described in U.S. Pat. Nos. 6,226,887 and 6,122,836 to Tenedini, wherein vacuum pumps are cycled off and on to regulate pressure.
Known freeze dryers incorporate many process control features to accomplish freeze drying, as described in U.S. Pat. Nos. 4,823,478 and 4,780,964 to Thompson. Features may include a cascading, multi-stage compression refrigeration system to achieve temperatures below −35° F., separate chambers for accumulating solid water from the sublimation process, temperature controllers programmed for a specific product, thermocouples to provide information to the temperature controller, and a programmable controller to receive instructions from an operator. An operator has a significant understanding of the freeze drying process in order to adequately set up the process controls. Solid state relays, switches, and digital and analog devices allow a desired “recipe” to control the freeze drying process.
A desire exists in freeze drying to control the rate of sublimation for a particular material. Heating the material too much in the vacuum chamber may thaw the material to a liquid or may generate too much gaseous water, overwhelming the vacuum pump and increasing the pressure too high for optimum moisture removal. For a given material, how it is prepared, its size, and its shape affect a given recipe and adds complication to the process. Accordingly, a desire exists to simplify freeze drying by reducing the effort involved in trial and error determination of freeze drying conditions and parameters.